It is by now well known in the water filtration field to utilize filtration tanks divided into a plurality of filter cells, one adjacent the other, and all containing a multi-layer or single layer arrangement of granular filter media such as sand, gravel and the like. In downward or gravity filtration systems of this type, water or other liquid containing suspended solid particles is introduced into the filtration tank from above, and clarified water is drawn off from a chamber (also referred to herein as a filtrate channel) either directly beneath, or adjacent and below the individual filter cells. In other words, in some instances, a common filtrate chamber extends beneath all of the filter bed cells. In other instances, the cell partitions extend below the cells to the bottom wall of the tank, and each individual cell has a port which permits the filtrate to flow out of the cell and into a common filtrate channel extending along side the tank.
During downward flow through the individual cells, particulate matter is entrapped within the layer or layers of granular filter media, but eventually, the particulate matter clogs the filter media, thereby impairing the filtering capability of the system. Thus, there is a need for periodic cleaning of the individual filter cells, typically by way of a backwash operation where filtered liquid is reverse flowed through the filter cells, one after the other, until the entire tank has been backwashed. It is also known to maintain such units in continuous operation during cleaning of individual cells, through the use of a traveling bridge device which moves from one filter cell to the next, backwashing individual cells while permitting the normal filtration process to continue in the remaining cells.
Examples of travelling bridge apparatus of this type may be found in U.S. Pat. Nos. 5,089,117; 4,988,439; 4,957,631; 4,859,330; 4,764,288; 4,617,131; 4,540,487; 4,486,307; 4,133,766; 3,984,326; 2,235,227; and 2,302,449. Typically, travelling bridge systems include an overhead carriage movable along tracks, guideways or the like, and carrying a backwash hood which may successively engage (or come into close proximity to) the open upper end of each filter cell. For a downflow or gravity type filter, after the backwash hood has been located in the prescribed manner, water or other previously filtered treatment liquid is generally introduced by a backwash pump into the cell from below, in a counterflow arrangement to the normal filtering direction. The backwash hood typically includes a suction head (i.e., a waste water pump) for drawing out fluid and debris flushed out of the filter cell as a result of the backwash. As the backwash of individual cells is completed, the travelling bridge moves to the next adjacent cell.
It is also known to combine backwash and rewash operations in a single traveling bridge system, and examples of this arrangement may be found in U.S. Pat. Nos. 4,540,487; 4,617,131; 4,764,288; and 5,089,117. In a rewash operation, water or other rewash liquid is passed through the filter media in the normal filtering direction, usually immediately after backwash.
The overall construction of the filtration tank of this invention is generally similar to that described in commonly owned U.S. Pat. No. 4,859,330, incorporated herein by reference. In that patent, a backwash/air scour system is disclosed, but the general arrangement of the tank, filter cells or compartments, and traveling bridge is similar. The present invention represents an improvement over the prior art backwash/rewash systems in that a single backwash pump located within the filtrate channel is utilized in combination with a cell shoe which incorporates both backwash and rewash heads. Moreover, both the backwash head and the rewash head of the sealing shoe are designed to continuously seal with the individual cell ports (referred to as backwash or rewash ports, depending on the operation) of the respective filter cells. At the same time, the backwash collection hood is also adapted to seal with adjacent partitions which define an individual cell. The ability to preclude contamination of the filtrate within the filtrate channel by reason of the sealing engagement of the shoe at the respective ports is important (particularly during rewash).
Another significant feature of this invention is the utilization of rewash liquid in the backwash of the cell adjacent and downstream (in the direction of movement of the bridge) of the cell being rewashed.
In an exemplary embodiment, the system includes a tank divided into a plurality of cells, each of which includes filter media supported above the bottom of the tank, such that filtrate below the filter media can drain into a common filtrate channel via individual outlet ports. The associated traveling bridge includes a collection hood which is adapted to seal against the partitions forming each cell, and serves to carry away the backwash water flowing upwardly through the cell during backwash. The bridge also carries a backwash pump which is submerged within the adjacent filtrate channel, and which supplies backwash water to each cell. The construction described immediately above is fairly conventional, noting the above identified prior U.S. patent documents. In a preferred arrangement in accordance with this invention, the submersible backwash pump is mounted in the adjacent filtrate (or effluent) channel to pressurize the filter cell plenum, and a backwash/rewash cell shoe mechanism segregates and seals the cells to be backwashed and rewashed from the filtrate channel, thereby preventing contamination of the filtrate in the channel while maintaining positive flow control.
Generally, during backwash, the traveling bridge will travel from its rest position until the backwash port on the shoe aligns with a first filter cell port. The backwash or collection hood is then sealed to the upper edges of the cell, and the cell is then backwashed for a preset length of time, with filtrate from the filtrate channel pumped into the cell in a direction counter to the normal filtration flow direction. Backwash liquid is removed via the collection hood. The bridge will then move to the next cell where the backwash head on the shoe is aligned with a next adjacent, or second cell port. At this time, the port of the first (already backwashed) cell will be aligned with the rewash head on the shoe. The bridge will remain in place until the backwash and rewash functions are completed on the second and first adjacent cells, respectively. Once the bridge has reached the second cell, rewash water is combined with filtrate channel liquid for backwashing, and both operations may be carried out simultaneously (or sequentially if desired). The bridge will then travel to the next or third cell, where backwash and rewash operations are carried out in the third and second cells, respectively. This procedure is repeated until each cell in the tank has been backwashed and rewashed.
More specifically, in the exemplary embodiment, the backwash/rewash shoe is equipped with a first sealing face which slides against and seals to a liner strip running the length of the tank and overlapping the filter cell outlet ports. Continuous sealing is accomplished by the shoe sealing face being kept in contact with the liner strip during the entire process by means of positive tension forcing the shoe against the liner strip, thereby eliminating leakage. This sealing of the shoe to the cell ports may be achieved through suitable conventional techniques.
In a simple, one-way system, a backwash supply valve is located in the conduit extending between the effluent channel and the inlet to the backwash pump. At the same time, the outlet line of the backwash pump is connected to the backwash head of the backwash/rewash shoe, with a backwash valve inserted between the pump outlet and the backwash port inlet (adjacent the filter cell outlet). Once the traveling bridge is in place, backwash is initiated by opening both the backwash supply valve and the backwash valve to create a path for backwash water to be pumped from the filtrate channel through the filter cell into the backwash hood and then to waste. When backwash is completed for this cell, the backwash supply valve and the backwash valve will close.
A rewash valve is located in the backwash pump outlet line beyond the branch containing the backwash valve. In the rewash mode, the recently backwashed cell is flushed in the normal downflow direction to remove any residual, suspended or particulate material such as bacteria or virus that may be left in the filter bed after backwashing. This is accomplished by a flow path created by the rewash head on the shoe being connected to the backwash pump suction. Since the rewash head of the shoe is connected to the pump suction, flow through the cell under rewash will exit through the backwash port and into the backwash pump and will be expelled with the backwash water during backwash of the adjacent downstream cell.
Thus, in accordance with the invention, since the backwash/rewash cell shoe includes both rewash and backwash heads adapted to align with adjacent cell ports or outlets, a trailing adjacent cell is rewashed during the backwash of the succeeding cell (in the direction of movement of the bridge), with rewash liquid from the trailing cell used as a component of the backwash liquid (along with filtrate from the filtrate channel) in the leading cell. Backwash and rewash on adjacent cells can also occur sequentially rather than simultaneously if desired.
It is recognized that rewash cycles can require more time than backwash cycles, and therefore, provisions must be made to handle the rewash water separately. In accordance with this invention, a valving arrangement is provided so that rewash liquid can be diverted around the cell which is being simultaneously backwashed. The diverted rewash can be recycled to the influent channel or to waste.
In a variation of the system in accordance with the invention, rewash heads are provided on either side of a backwash head in the backwash/rewash shoe, thus enabling the backwash/rewash operations to be carried out with the bridge moving in either of two opposite directions.
Accordingly, in one aspect, the invention relates to a method of cleaning a plurality of side-by-side gravity flow filter cells, each having an outlet port in communication with a filtrate channel common to all of the cells in an automatic backwash/rewash system, comprising the steps of subjecting each compartment, sequentially, to a backwashing operation and then a rewashing operation utilizing a backwash/rewash shoe having adjacent backwash and rewash heads therein slidably movable along the cells and sequentially and sealingly engageable with each outlet port wherein, during backwashing, a backwash pump causes backwash liquid to flow through the backwash head and a first of the filter cell ports and then upwardly through a first of the cells in a direction opposite to a normal filtration flow direction, and wherein during rewashing, rewash liquid is drawn downwardly through the first cell in the normal filtration flow direction through said rewash head, and then to said backwash pump; and for any given cell after a first of the plurality of cells, utilizing at least a portion of the rewash liquid from a rearwardly adjacent filter cell as backwash liquid in the given cell.
In another aspect, the invention relates to an apparatus for cleaning a filtration tank having a plurality of side-by-side gravity flow filter cells, each having an outlet port in communication with a filtrate channel common to all of the cells in an automatic backwash/rewash system, by subjecting each compartment, sequentially, to a backwashing operation and then a rewashing operation, the apparatus comprising a backwash pump utilized to pump backwash liquid through a first of the filter chamber ports to flow upwardly through a first of the compartments in a direction opposite to a normal filtration flow direction; and a backwash/rewash shoe including a backwash head and a rewash head each adapted for respective sealing engagement with a pair of adjacent outlet ports, the rewash head connected to an inlet of the backwash pump, and the backwash head connected to an outlet of the backwash pump.
Other objects and advantages of the invention will become apparent from the detailed description which follows.